Fixes from polymer.dart API review

pkg/polymer/test/unbind_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library polymer.test.unbind_test;
 
-import 'dart:async' show Future;
+import 'dart:async' show Future, scheduleMicrotask;
 import 'dart:html';
 
 @MirrorsUsed(targets: const [Polymer], override: 'polymer.test.unbind_test')
 import 'dart:mirrors' show reflect, reflectClass, MirrorSystem, MirrorsUsed;
 
 import 'package:polymer/polymer.dart';
-import 'package:polymer/platform.dart' as Platform;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 @CustomTag('x-test')
 class XTest extends PolymerElement {
   @observable var foo = '';
   @observable var bar;
 
   bool forceReady = true;
   bool fooWasChanged = false;
@@ skipping 40 matching lines @@
 
 // TODO(jmesserly): fix this when it's easier to get a private symbol.
 final unboundSymbol = reflectClass(Polymer).variables.keys
     .firstWhere((s) => MirrorSystem.getName(s) == '_unbound');
 
 _unbound(node) => reflect(node).getField(unboundSymbol).reflectee;
 
 unbindTests() {
   var xTest = document.query('x-test');
   xTest.foo = 'bar';
-  Platform.flush();
+  scheduleMicrotask(Observable.dirtyCheck);
 
   return delay(null).then((_) {
     expect(_unbound(xTest), null, reason:
         'element is bound when inserted');
     expect(xTest.fooWasChanged, true, reason:
         'element is actually bound');
     xTest.remove();
   }).then(delay).then((_) {
     expect(_unbound(xTest), true, reason:
         'element is unbound when removed');
     return new XTest();
   }).then(delay).then((node) {
     expect(_unbound(node), null, reason:
         'element is bound when not inserted');
     node.foo = 'bar';
-    Platform.flush();
+    scheduleMicrotask(Observable.dirtyCheck);
     return node;
   }).then(delay).then((node) {
     expect(node.fooWasChanged, true, reason: 'node is actually bound');
     var n = new XTest();
     n.cancelUnbindAll();
     return n;
   }).then(delay).then((node) {
     expect(_unbound(node), null, reason:
         'element is bound when cancelUnbindAll is called');
     node.unbindAll();
     expect(_unbound(node), true, reason:
         'element is unbound when unbindAll is called');
     var n = new XTest()..id = 'foobar!!!';
     document.body.append(n);
     return n;
   }).then(delay).then((node) {
     expect(_unbound(node), null, reason:
         'element is bound when manually inserted');
     node.remove();
     return node;
   }).then(delay).then((node) {
     expect(_unbound(node), true, reason:
         'element is unbound when manually removed is called');
   });
 }